The field of the invention is magnetic resonance imaging (MRI) methods and systems. The invention is a new computational method for the formation of MR images using the data acquired by the multiple receiver channels available on standard MRI.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant of the nucleus). Nuclei which exhibit this phenomena are referred to herein as “spins”.
When a substance such as human tissue is subjected to a uniform, static magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal that is emitted by the excited spins after the excitation signal B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (“NMR”) phenomenon is exploited.
When utilizing NMR to produce images, a technique is employed to obtain NMR signals from specific locations in the subject. Typically, the region that is to be imaged (region of interest) is scanned by a sequence of NMR measurement cycles which vary according to the particular localization method being used. The resulting set of received NMR signals, or “views”, are digitized and processed to reconstruct the image using one of many well known reconstruction techniques. To perform such a scan, it is, of course, necessary to elicit NMR signals from specific locations in the subject. This is accomplished by employing magnetic fields (Gx, Gy, and Gz) that have the same direction as the polarizing field B0, but which have a gradient along the respective x, y and z axes. By controlling the strength of these gradients during each NMR cycle, the NMR signal can be spatially “encoded”, i.e., the spatial distribution of spin excitation can be controlled and the location of the resulting NMR signals can be identified.
The present invention will be described with reference to a variant of the Fourier transform (FT) imaging technique, which is frequently referred to as “spin-warp”. The spin-warp technique is discussed in an article entitled “Spin Warp NMR Imaging and Applications to Human Whole-Body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of NMR spin-echo signals to phase-encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (ΔGy) in the sequence of views that are acquired during the scan to produce a set of NMR data from which an entire image can be reconstructed.
General fast imaging methods in MRI attempt to acquire the maximum amount of MR data in the given interval of time (for reference, see M. S. Cohen and R. M. Weisskoff, “Ultra-fast imaging,” Magn Reson Imaging, vol. 9, pp. 1-37, 1991). To gain acceleration in the imaging process, fast imaging methods typically rely on advanced MR hardware such as fast gradients, multiple RF receiver channels (coils), and corresponding imaging schemes such as fast pulse-sequences and parallel MRI schemes (described below).
Fast pulse sequences and the hardware requirements to support them are well known and widely used (for reference, see M. Bernstein, K. King and X. Zhou, “Handbook of MRI Pulse Sequences.” Elsevier, 2004, pp. 1040). However, these methods are limited owing to physiological limitations, e.g., peripheral nerve stimulation caused by fast switching gradient pulses. Furthermore, parallel imaging techniques such as the present invention can take advantage of any such methods and hence enjoys their benefits.
A recent technique used to shorten scan time is referred to generally as “parallel magnetic resonance” (PMR) imaging and is sometimes referred to as “partially parallel acquisition” or “partial parallel MRI”. Using MR scanner equipped with multiple receiver channels and a set of RF receiver coils (also referred to as phased-array coils), PMR techniques use spatial information from the array of RF detector coils to substitute for the encoding which would otherwise have to be obtained in a sequential fashion using field gradients and RF pulses. In conjunction with recently developed data acquisition technologies, parallel MRI (PMRI) can significantly accelerate the data acquisition without increasing gradient switching rates or RF power deposition. Several variations of PMRI schemes have been reviewed in the following two articles: M. Blaimer et al., “SMASH, SENSE, PILS, GRAPPA: how to choose the optimal method,” Top. Magn. Reson. Imaging, vol. 15, pp. 223-236, August 2004; and A. C. S. Brau et al., “Comparison of reconstruction accuracy and efficiency among autocalibrating data-driven parallel imaging methods,” Magnetic Resonance in Medicine, vol. 59, pp. 382-395, 2008.
Three such parallel imaging techniques that have recently been developed and applied to in vivo imaging are SENSE (Sensitivity Encoding; Pruessmann et al., Magnetic Resonance in Medicine Vol. 42, p.952-962, 1999), SMASH (simultaneous acquisition of spatial harmonics; disclosed in U.S. Pat. No. 5,910,728 issued on Jun. 8, 1999) and GRAPPA (generalized autocalibrating partially parallel acquisitions; M. A. Griswold et al., Magn. Reson. Med., vol. 47, pp. 1202-1210, June 2002; disclosed in U.S. Pat. No. 6,841,998 issued on Jan. 11, 2005). These techniques include the parallel use of a plurality of separate receiving coils, with each coil having a different sensitivity profile. The combination of the separate NMR signals produced by these coils enables a reduction of the acquisition time required for an image (in comparison with conventional Fourier image reconstruction) by a factor which in the most favorable case equals the number of the receiving coils used as explained by Pruessmann et al., Magnetic Resonance in Medicine Vol. 42, p. 952-962, 1999. For pulse sequences that execute a rectilinear trajectory in k-space, parallel imaging techniques usually reduce the number of phase encoding steps in order to reduce imaging time, and then use the coil sensitivity information to make up for the loss of spatial information.